This invention relates to a battery unit comprising a chargeable electric cell (secondary battery) such as a lithium ion cell and, in particular, to an overcharge protection circuit for protecting the secondary battery from overcharge.
Among various types of chargeable electric cells (secondary batteries), a lithium ion cell is particularly weak against overdischarge or overcharge. In this connection, it is essential to provide a battery protection circuit (battery protection integrated circuit (IC)) for detecting an overdischarge condition and an overcharge condition to protect a secondary battery from the overdischarge condition and the overcharge condition. For this purpose, the battery protection IC has two mechanisms (two functions): an overdischarge prevention mechanism (an overdischarge protection function) and an overcharge prevention mechanism (an overcharge protection function). In addition, the battery protection IC may detect an overcurrent condition during discharge of the secondary battery to protect the secondary cell from the overcurrent condition. In this event, the battery protection IC may have three mechanisms (three functions): an overcurrent prevention mechanism (an overcurrent protection function) as well as the overdischarge prevention mechanism (the overdischarge protection function) and the overcharge prevention mechanism (the overcharge protection function).
A battery unit comprising such as a battery protection IC is called a battery pack in the art. The battery pack has a pair of pack output terminals for selectively connecting a charger or a load therewith. In the pair of pack output terminals, one is a positive electrode terminal and another is a negative electrode terminal.
When the secondary battery is put into the overdischarge condition, it is necessary to stop a discharging operation and to connect the charger with the pack output terminals to charge the secondary battery. Generally, an overcharge protection circuit is for protecting a secondary battery by controlling, by turning a charge control switch on and off, a charging current flowing through the secondary battery from the charger. The secondary battery is connected between a power source terminal and a ground terminal.
In the manner which will later be described in conjunction with FIGS. 1-3, a conventional overcharge protection circuit comprises an overcharge detection section and an overcharge blind time setting circuit connected to the overcharge detection section. The overcharge detection section detects whether or not the secondary battery is put into an overcharge condition to produce an overcharge detected signal on detection of the overcharge condition. The overcharge blind time setting circuit has an overcharge blind time setting terminal and includes a capacitor connected between the overcharge detection blind time setting terminal and the ground terminal. Responsive to the overcharge detected signal, the overcharge blind time setting circuit produces an OFF control signal for turning the charge control switch off after a lapse of a predetermined overcharge detection blind time interval defined by a capacitance value of the capacitor.
In the manner which is described above, the conventional overcharge protection circuit monitors a battery voltage of the secondary battery and prohibits charging, when the battery voltage is equal to the overcharge detection voltage or more, by turning the charge control switch off after a lapse of the predetermined overcharge detection blind time interval set in the overcharge detection blind time setting terminal.
However, in a case where the charger is an abnormal one so as to flow, as the charging current, an excessive current (which has a current value, for example, of ten amperes or more), problem arises in the conventional overcharge protection circuit as follows.
More specifically, in this case, the battery voltage of the secondary battery becomes the overcharge detection voltage or more immediately after the secondary battery is charged by the charger. Nevertheless, in the conventional overcharge protection circuit, the excessive current continues to flow from the charger to the secondary battery during the overcharge detection blind time interval set in the overcharge detection blind time setting terminal. As a result, it is feared that the excessive current causes damage to the charge control switch and so on during the overcharge detection blind time interval.
It is therefore an object of the present invention to provide an overcharge protection circuit which is capable of preventing damage to a charge control switch when a charger is in an abnormal state and an excessive current is flowing as a charging current.
Other objects of this invention will become clear as the description proceeds.
On describing the gist of an aspect of this invention, it is possible to be understood that a method is for protecting a secondary battery by controlling, by turning a charge control switch on and off, a charging current flowing through the secondary battery from a charger.
According to the aspect of this invention, the above-mentioned method comprises the steps of detecting whether or not the charging current is an excessive current, and of immediately turning the charge control switch off when the charging current is the excessive current.
On describing the gist of another aspect of this invention, it is possible to be understood that a secondary battery protection circuit is for protecting a secondary battery by controlling, by turning a charge control switch on and off, a charging current flowing through the secondary battery from a charger.
According to the other aspect of this invention, the above-understood secondary battery protection circuit comprises a detecting arrangement for detecting whether or not the charging current is an excessive current. A switch off arrangement is for immediately turning the charge control switch off when the charging current is the excessive current.
On describing the gist of still another aspect of this invention, it is possible to be understood that an overcharge protection circuit is for protecting a secondary battery by controlling, by turning a charge control switch on and off, a charging current flowing through the secondary battery from a charger connected between a positive electrode terminal and a negative electrode terminal. The secondary battery is connected between a power source terminal and a ground terminal to generate a battery voltage.
According to still another aspect of this invention, the above-understood overcharge protection circuit comprises an overcharge detection section for detecting whether or not the second battery is put into an overcharge condition. The overcharge detection section produces an overcharge detected signal on detection of the overcharge condition. Connected to the overcharge detection section, an overcharge blind time setting circuit has an overcharge blind time setting terminal and includes a capacitor connected between the overcharge blind time setting terminal and the ground terminal. Responsive to the overcharge detected signal, the overcharge blind time setting circuit produces an OFF control signal for turning the charge control switch off after a lapse of a predetermined overcharge detection blind time interval defined by a capacitance value of the capacitor. An excessive current detecting arrangement detects whether or not the charging current is an excessive current. A forcibly discharging arrangement forcibly and immediately discharges electrical charges accumulated in the capacitor when the excessive current is detected by the excessive current detecting arrangement, thereby immediately turning the charge control switch off upon detection of the excessive current without waiting for the lapse of the predetermined overcharge detection blind time interval.